1. Field of the Invention
The present invention relates to modulation of a series of m-bit information words into a modulated signal and demodulation of the modulated signal, and more particularly, to a method of generating a run length limited (RLL) code which uses the less number of bits for a code word and efficiently suppresses the DC component of a code word stream, and a method of modulating/demodulating the RLL code for use in optical recording and/or reproducing devices requiring high density recording and/or reproducing.
2. Description of the Related Art
In optical recording and/or reproducing devices, a run length limited (RLL) code represented by (d, k, m, n) is widely used for modulating original information into a signal appropriate for an optical disk and demodulating the signal reproduced from the optical disk into the original information.
In the RLL code represented by (d, k, m, n), main conversion of data is carried out using the main conversion table of FIG. 1A. The code groups 1 and 4 in the main and sub conversion tables of FIGS. 1A and 1B are divided by the number of lead zeros (LZ), while the code groups 2 and 3 are also exclusively divided by referring to a predetermined bit of the code word and determining whether the bit is "0", or "1". That is, no code words in the code groups 1 and 4 are the same, and no code words in the code groups 2 and 3 are the same. The main and sub conversion tables of FIGS. 1A and 1B are suggested in U.S. Pat. No. 5,790,056, "Method of converting a series of m-bit information words to a modulated signal, method of producing a record carrier, coding device, device, recording device, signal, as well as a recorded carrier."
The RLL code uses a (d, k)-constraint method in which code words of a code group select a next code group in compliance with the (d, k) constraint. Using the sub conversion table of FIG. 1B, suppression of the DC component in a code word stream can be carried out when data less than or equal to 87 is input. The code words of the code groups in the sub conversion table are allocated by using code words not included in the main conversion table, each of the code words of the code groups in the sub conversion table having a code word sum value (CSV), a parameter representing the direct current (DC) value in a code word, which is opposite in sign to CSVs of corresponding code words having the same decoded value in the main conversion table, in order to suppress the DC component.
In the code groups 1 and 4 in the main conversion table and the sub conversion table of FIGS. 1A and 1B, a code group advantageous to suppression of the DC component can be selected if the (d, k) constraint of the run length is not violated. However, the code words are allocated without considering the CSV signs of code words which are included in the code groups 1 and 4 and the characteristics of an INV parameter, proposed by the present invention, which predicts the transition direction of a next code word by determining whether the number of "1"s in a code word is an odd number or an even number. Therefore, it is not appropriate to generate a code conversion table to convert an 8-bit information word into a 15-bit code word, separately from the conventional code (2, 10, 8, 16) of FIGS. 1A and 1B, which converts an 8-bit information word into a 16-bit code word.
In addition, for data less than or equal to 87, a code word can be selected from both the main conversion table and the sub conversion table in order that their CSV signs are opposite, so that the DC component can be suppressed. However, because the code words of the sub conversion table are newly allocated code words which are not included in the main conversion table, as much code words as the code words in the sub conversion table are additionally required after all, which imposes a limitation in reducing the number of bits of a code word.
Also, the code words in the sub conversion table are allocated to have opposite signs to those of corresponding code words having the same decoded value in the main conversion table, but the characteristics of INV which is a code word parameter proposed by the present invention are not considered for allocation, which causes a problem in that the possibility of DC suppression cannot be maximized when a modulated code with a smaller number of code words is used in DSV control which is carried out in a predetermined look-ahead method.